This invention relates generally to plasma-enhanced chemical vapor deposition (PECVD) for applying a coating on a substrate, and more specifically to a method for stabilizing the internal surface of a PECVD process chamber using a plasma.
In the formation of integrated circuits (IC""s), thin coatings or films containing metal and metalloid elements are often deposited upon the surface of a substrate, such as a semiconductor wafer. Such thin films are deposited to provide conductive and ohmic contacts in the circuits formed on the substrate and between the various components of an IC. For example, a conductive thin film might be applied to the exposed surface of a contact or via hole on a semiconductor wafer, with the film passing through the insulative layers on the wafer, to provide plugs of conductive material for the purpose of making electrical interconnections across the insulating layers.
One well known process for depositing thin metal films is chemical vapor deposition (CVD), in which a thin film is deposited as a result of chemical reactions between various deposition or reactant gases at the surface of the substrate. In CVD, reactant gases are pumped into proximity with a substrate inside a reaction chamber. The reactant gases subsequently react at the substrate surface, resulting in one or more reaction by-products which form a film on the substrate surface. Any by-products remaining in the chamber after the deposition are removed from the chamber.
One variation of the CVD process which is widely utilized is a plasma-enhanced CVD process or PECVD process, in which one or more of the reactant gases is ionized into a gas plasma to provide energy to the reaction process. PECVD may be desirable, for example, for lowering the temperatures which are usually necessary for a proper chemical reaction with standard CVD. In PECVD, electrical energy is delivered to the reactant gas or gases to form and sustain the plasma. For one such PECVD process, the susceptor or support containing the substrate and a planar element in the processing space, such as a planar gas supply element, are electrically biased to operate as opposing RF electrodes for energizing one or more of the reactant gases into an ionized plasma. Such a method is commonly referred to as a parallel plate method because the susceptor and the biased planar element are maintained generally parallel to one another to simulate biased electrical plates with the substrate positioned therebetween and generally parallel to the biased elements.
The reactant gases for CVD and PECVD processes are delivered to the processing space and substrate through a gas delivery system which provides the proper flow and distribution of the gases for the CVD process. Generally, such gas delivery systems contain gas-dispersing elements in the reaction chamber, such as gas injector rings or flat showerheads, which spread the entering reactant gases around the processing space to insure a uniform distribution and flow of the gases proximate the substrate. Uniform gas distribution and flow is desirable for a uniform and efficient deposition process, a dense plasma, and a uniformly deposited film.
One notable PECVD method involves the deposition of thin films of titanium and titanium-containing layers onto silicon substrates. Generally, for such a method, a plasma comprising TiCl4, H2, and Ar is utilized. Such a deposition process is described in U.S. Pat. Nos. 5,628,829; 5,665,640; 5,567,243; and 5,716,870, which patents are incorporated herein by reference in their entirety. During the deposition process, TiCl4, which is partially reactive, condenses onto the walls of the processing chamber. The partially reacted TiCl4 may be TiCl2 or TiCl3 or hydrogenated versions of these, such as H2TiCl2 or HTiCl3. The condensation generally occurs due to the relatively low temperature of the chamber walls with respect to the susceptor and substrate.
Specifically, the susceptor and substrate are maintained at a temperature in the range of about 400xc2x0 C. or above, and usually around 600xc2x0 C. Such a processing temperature supports a complete reduction of the TiCl4 gas and the subsequent deposition of titanium metal. However, the processing chamber internal walls are generally not as hot as the substrate and may be maintained at a temperature in the range of 80-200xc2x0 C., and usually around 175xc2x0 C. The wall temperature does not support complete decomposition of the TiCl4, and thus, titanium sub-chlorides (TiClx where x less than 4) are deposited onto the wall surfaces due to their low vapor pressure.
Such sub-chlorides are deposited onto the internal chamber walls as a powder, and they remain in the powder form as long as the processing chamber remains under vacuum pressure. The sub-chloride powder tends to become oily when the processing space inside of the chamber is exposed to atmosphere. This is generally due to moisture absorption, because the powder has a hygroscopic nature. An analysis of the powder has shown that it generally contains a mixture of TiCl2 and TiCl3.
Two specific problems are caused by the deposition on the reaction chamber walls. First, the deposition actually adheres very poorly to the walls. This leads to flaking of the residue from the walls and subsequent particle contamination on the substrate. The powder generally cannot be removed by conventional dry etching techniques (with the exception of ClF3). Therefore, the processing chamber generally must be opened approximately every 200 deposition cycles for manual cleaning of the powder.
A second problem is that the deposited powder consists largely of titanium sub-chlorides, which have a high vapor pressure relative to the deposition pressure. Therefore, some of the sub-chlorides are volatilized during the deposition process. This volatilized material then diffuses to the substrate, and participates in the film deposition reaction. It has been observed that, for deposition onto silicon substrates, where the product of the deposition reaction is a titanium silicide film, the sub-chlorides from the processing chamber wall lead to a net increase in the amount of film deposited. Therefore, the deposition of the film is somewhat uncontrolled, which is an undesirable characteristic. Using TiCl2 as an example, the following reaction occurs:
TiCl2+2Hxe2x86x922 less than Ti(s)+2HCl
The deposited titanium quickly reacts with the underlying silicon from the substrate to form titanium silicide, albeit in a somewhat uncontrolled fashion.
In order to overcome the effects of the powder residue on the chamber walls, an ammonia plasma treatment has been devised. This treatment is described in greater detail in U.S. Pat. No. 5,593,511 which is incorporated herein by reference in its entirety. The treatment stabilizes the titanium sub-chlorides by reacting them with ammonia and converting them to titanium nitride. The titanium nitride forms as an adherent film on the processing chamber wall. It does not cause particle contamination problems and does not contribute to the film deposition reaction on the substrate. However, it does cause other problems, and thus there is still a need for a suitable solution to address the problems associated with the deposition of titanium sub-chlorides in the processing chamber.
The specific problem involving titanium nitride is that the film forms on the process chamber wall and also on the electrical insulator which isolates the RF electrode from the reaction chamber walls. The effect is to create a conducting path from the RF electrode to the grounded reaction chamber walls. This problem is described in U.S. patent application Ser. No. 09/153,128, filed Sep. 15, 1998, entitled xe2x80x9cApparatus and Method for Electrically Isolating an Electrode in a PECVD Process Chamber,xe2x80x9d which describes the use of serrations in the insulator to increase the length of the path to ground. Although this method is effective, it only increases the time until a short circuit may occur due to the titanium nitride buildup.
Therefore, it is desirable to reduce the effect of the titanium sub-chlorides altogether. To that end, it is one objective of the present invention to reduce, and possibly eliminate, the buildup of titanium sub-chlorides on the processing chamber wall. The invention will become more readily apparent from the Detailed Description and further disclosure below.
The present invention utilizes a method which stabilizes the internal surface of a PECVD process chamber, provides a more stabilized deposition process, yields a cleaner chamber, and reduces the possibility of a biased showerhead shorting to ground, such as through the process chamber.
The invention comprises a process including the steps of positioning a substrate within the processing space of a processing chamber and depositing a material layer on the substrate using plasma enhanced chemical vapor deposition. In one embodiment of the invention, titanium reactant gas is utilized and a titanium-containing layer is deposited onto a substrate. After deposition, the substrate is removed from the processing chamber and the processing space is purged, such as with a hydrogen/argon mixture. The purging step may be repeated several times. Thereafter, a gas mixture is introduced into the processing space. The gas mixture contains oxygen, and in one embodiment, the mixture is an oxygen/argon mixture. For example, the mixture might be 5% oxygen and 95% argon. However, other mixtures including a greater or lesser percentage of oxygen would be useful as well in the present invention.
RF energy is coupled to the processing chamber to excite or maintain an oxygen-containing plasma created from the gas mixture. The oxygen-containing plasma is sustained for a predetermined amount of time within the processing space. For example, a 60 second oxygen-containing plasma has been shown to be useful in achieving results with the present invention. In one aspect of the present invention, the oxygen-containing plasma might be followed by an argon/hydrogen purge of the processing space.